a. Field of the Invention
The present invention relates to satellite communication systems that use phase shift keyed (PSK) or differential phase shift keyed (DPSK) signalling. In particular, it relates to mobile satellite communications receivers which use PSK or DPSK signalling.
b. Problems in the Art
Many modulation schemes, e.g., PSK, DPSK, frequency shift key (FSK), single side band (SSB)-analog, and amplitude modulation (AM)-analog, require an accurate estimate of the system's carrier frequency for proper demodulation of the received signal. Demodulation of PSK and DPSK signals is particularly susceptible to a receiver's tuned frequency being offset from the desired carrier frequency. In addition to the carrier frequency, coherent-demodulation also requires estimation of the carrier phase. Generally, a Costas or squaring loop (which are more complex forms of the basic phase locked loop) provides the required estimate of carrier frequency and phase for coherent demodulation of PSK or DPSK signals. Unfortunately, Costas and squaring loops have only a limited capture range. To provide adequate loop demodulation performance, especially in low signal to noise ratio applications, the capture ranges of Costas and squaring loops must be limited. This is because the phase noise in the loop is proportional to the loop bandwidth which also governs the capture range. Consequently, Costas and squaring loops' capture ranges are often aided by supplemental means. One such means is spectral analysis of the received signal.
Spectral analysis of a mobile satellite system's received signal is sometimes employed to aid a loop in carrier acquisition. Spectral analyses are performed and averaged, yielding a coarse estimate of the receiver's frequency offset from the carrier. Although this approach will typically provide a frequency estimate within the loop's capture range, this approach requires a great deal of processing power. Applications involving low signal to noise ratio require even more processing because the desired signal is masked by noise and must be extracted using several Fast Fourier Transforms (FFTs) followed by averaging and some form of feature extraction. Processing power requirements severely limit the applicability of this approach to aiding loop acquisition.
Another approach to aiding phase locked loop (or PLL) acquisition employs a frequency locked loop to initially provide the primary correction and drive to the phase locked loop's VCO, when the offset between the frequency of the desired signal and that of the VCO is relatively large, and using the PLL's inherent capture ability when the offset is small.
In its most basic configuration, a Costas loop's capture range is comparable (about 1.5 times greater) to its single-sided loop noise bandwidth. One can extend the capture range of the basic loop considerably beyond the loop noise bandwidth by creating a component in the VCO's input drive that was proportional to the instantaneous frequency offset between the VCO's output and the input signal. Adding an automatic frequency control (AFC) loop around the Costas loop provides the desired VCO drive signal.
Although the addition of an AFC loop around the Costas loop increases the composite loop's capture range, the acquisition time of the composite loop is long because the AFC loop bandwidth has to be small relative to that of the Costas loop. This is so that the AFC loop's contribution to the VCO phase noise is small. In fact, in Cahn's work (referenced above and incorporated by reference herein), the AFC loop bandwidth is limited to 1/10 of the Costas loop bandwidth, at most. For slowly varying signals, in applications where channel utilization is not extremely critical, this bandwidth penalty is tolerable and, many times, the increase in capture range more than compensates for the sacrifice in loop bandwidth. However, for many applications, loop bandwidth is critical and this approach is untenable.
In particular, mobile satellite communication systems operate with low signal to noise ratios. Furthermore, the mobile satellite receiver typically "hops" its tuned frequency over a range of uncertainty until it locates its desired carrier frequency. This "hopping" effectively creates a step-function input to the carrier acquisition subsystem. A limited loop bandwidth constricts an acquisition system's response to a rapidly changing input like a step-function.
Satellite time is extremely expensive, and anything that reduces a satellite communication system's acquisition time enhances the utilization of a valuable resource. A technique for improving channel utilization through the reduction of acquisition time greatly improves the carrier acquisition system's value. Achieving this improvement is especially difficult when faced with the obstacles presented by a step function input in combination with a limited loop bandwidth.
It is therefore an object of the present invention to provide means and a method for minimizing the search time required for a satellite communication acquisition system to acquire an associated communication channel.
It is also an object of the present invention to minimize the time required for a mobile satellite receiver to reacquire a satellite signal after interruption. Such systems require narrow tracking bandwidths in order to provide good demodulation performance at low signal-to-noise ratios. Fading and blocking of signals cause mobile satellite systems to suffer much more frequently from the loss of carrier than stationary satellite systems. Rapid reacquisition is therefore especially important to mobile satellite systems.
It is a further object of the present invention to provide means and a method for acquisition and tracking of a satellite communication signal which is cost effective.
Some carrier acquisition subsystems use a computation intensive approach to expand the capture range of a phase locked loop. In these systems, high speed, relative costly processors continuously run Fast Fourier Transforms in the background. It is therefore a further object of the system of the present invention to eliminate the expense associated with high performance processing components by obviating the need for continuous Fast Fourier Transform computation.
It is a further object of the present invention to provide means and a method for extending the capture range of a PLL (phase locked loop), so that it can pull in signals with large frequency offsets, without degrading the signal to noise ratio in the tracking mode.
Other objects, features, and advantages of the invention will become apparent with reference to the accompanying specification and claims.